Method of anchoring a nitrocellulose base surface coating to a cellulose base sheet



Dec. 5, 1961 L. HAGAN ETAL METHOD OF ANCHORING A NITROCELLULOSE BASESURFACE COATING TO A CELLULOSE BASE SHEET Filed NOV. 28, 1958REGENERATED BASE SHEET FORMATION SOFTENI N6 8. BASE SHEET ANCHORINGAQUEOUS ALDEHYDE AND POLYMERIZED ETHYLINIMINE AQUEOUS POLYHYDRIC ALCOHOLSOFT ENER BATH LACQUER COMPOSITION FILM PRODUCT WITH ANCHO RED COATINGIN VEN TORS LAMONT HAGAN VINCENT D.CELLENTANO ATTORN United StatesPatent The subject invention relates to a composition useful inanchoring surface coatings to base surfaces, to the meth- 0d ofpreparing the composition, and to the surface coated article formedthereby.

In recent years, transparent sheet wrapping material comprising a basesheet of softened, regenerated cellulose having a thin coating of amaterial containing nitrocellulose as a principal ingredient on itssurfaces, has becomean important article of commerce. Such coatings maycontain a variety of ingredients to impart special properties such asresistance to moisture, low permeability to water vapor and the like, tothe finished article. The

manufacture of a typical non-fibrous base sheet is describedin PatentNo. 1,548,864 (Brandenberger). Coating ofthis base sheet with typicalcoating compositions is described in Patent Nos. 1,737,187, 1,826,697,2,147,- 180, and others.

When products containing considerable water, as for 7 example cheese,and certain vegetables, are wrapped in a coated transparent wrappingmaterial such as those referred to above, the surface coating separatesor loosens in a short time thus impairing the appearance andeffectiveness of the wrapping. The loosening or separation.

usually occurs in a few minuta when the material wrapped has a highcontent of available water, or when the wrapping material iscontinuously exposed to a high relative humidity. Such a wrappingmaterial i accordingly said to have low resistance to moisture.

Another problem which has existed in the use of surface coatedtransparent wrapping materials formed with a regenerated cellulose baseis that there is a tendency for a thermal seal, formed by placing twolayers of the material together and heating under slight pressure, tobecome separated due to ambient moisture conditions. This problem hasexisted for packages which are wrapped in a transparent regeneratedcellulose base wrapping material in the conventional manner, as forexample, by folding and overlapping the edges and heating the overlappedportions under slight pressure to seal the layers of wrapping togetherand to secure the wrap about the package. Under high humidityconditions, the seal formed between portions of the Wrapping has becomeloosened due to the effect of moisture on the cellophane and thewrapping has become separated from the article enclosed. Inasmuch as theseal bond is formed largely between the coatings on the contactedsurfaces of the sheets, the failure of the coatings to adhere to thebase sheets can result in the same loss of wrap as a failure of the bondformed by the seal itself.

A problem which has been encountered in the formation of softened,regenerated cellulose pellicles with strong ly anchored or adherentsurface coatings is that the anchoring agents, used to enhance theadherence, tend to react with the'material used in forming the pellicleand to precipitate or condense to an undesirable degree. Thus, it hasbeen the practice to include an anchoring agent in the softener bathused in imparting the desired concern tration of softener to the basesheet. In this way, the anchoring agent is incorporated into the basesheet along with the softener and the surface coating, which issubsequently applied to the base sheet, is strongly retained thereon.

A problem sometimes encountered in following this practice is theproblem of reaction between the anchoring agent included in the softenerbath and the sulfate ion carried over into this bath from the earlierprocess steps used in forming the base sheet. For example, where acondensible resin is incorporated in the softener bath, the action ofthe sulfate ion on this resin may be sufficient to cause itscondensation to such a degree that the anchorage enhancing value of theresin in the bath is lost. This leads to a degree of irregularity in theproperties of the coated sheet, and in the more severe case, a loss ofthe contents of the softener bath due to the necessity for discardingthem.

Accordingly, one object of the subject invention is to provide ananchoring agent capable of strongly anchoring surface coatings to basesurfaces.

Another object of the invention is to provide an anchoring agent whichmay be employed in a softener bath Without danger of reaction with thecontents of the bath.

Still another object is to provide a composition which induces stronganchorage between base surfaces and surface coatings formed thereon. Afurther object is to provide an article having a strongly anchoredsurface coating thereon. Still further objects will be in part apparentand in part pointed out herein after.

-In one of its broader aspects, the objects of the invention areachieved by reacting formaldehyde with polymerized ethylenimine andanchoring the surface coating to the base surface with this product.rower aspects, the objects of the invention are achieved by forming anaqueous solution of a softener and the reaction product of formaldehydeand polyethylenimine, impregnating a regenerated cellulose sheet withthis composition, drying the base sheet, coating said sheet with alacquer composition containing nitrocellulose predominantly, andcontaining an acid selected from the group consisting of itaconic,aoonitic, citric, lactic, acrylic, and methacrylic, heating the coatedbase sheet to evaporate the solvent and anchor the resultant coating tosaid base Example I A solution of polymerized ethylenimine in 1 kilogramof an aqueous glycerol solution containing 10% by weight of glycerol wasprepared by dissolving therein 4.3 grams of an aqueous solutioncontaining 50% solids of the ethylenimine therein. To this solution 8.1grams of 37% formaldehyde in water were added. The bath was observed tobe slightly cloudy. A specimen of regenerated cellulose gel film wasdipped into the bath until it was completely wet with the solution. Theexcess liquid was removed from the surfaces. The specimen was againdried and then surface coated with a thin layer of lacquer Vcomposition.

The coated base sheet was then heated toevaporate the solvent and anchorthe surface coatings formed.

.T he coated cellophane was then tested by immersion in water at C. Thecoating remained intact on the base sheet for minutes.

Example I] Substantially the same procedure of solution prepara- In oneof its nar- V hyde in water to the solution. 1

Y A. softener bath containing l% byweight of glycerol.

Example III A solution was formed by heating 17.2 grams of ethyleneglycol with 4.3 grams of a 50% aqueous solution of polyethylenimine. Tothis solution 8.1 grams of 37% formaldehyde was added at roomtemperature and asolution resulted. A softener bath containing less than1.0% by weight of the reaction product of formaldehyde andpolyethylenimine and byweight of glycerol was formed.

A cellulose sheet was impregnated with this solution, dried, and coatedas described above. A sample of the coated film was immersed in water at75 C., and it was observed that the coating did not disengage from thebase sheet for a period of two hours.

Where this same procedure of solution preparation was followed by usingonly 8.6 grams of ethylene glycol in place of 17.2 grams, an insolublegel was formed. Accordingly, the preferred procedure is to use thelarger amount of ethylene glycol.

Example IV Asolution was formed by heating 17.2 grams ethylene glycol,17.2 grams of'water, and 4.3 grams of polyethylenimine together at roomtemperature. This was followed by the addition of 8.1 grams of 37%formaldeand less than 1.0% by weight of the reaction product offormaldehyde and polyethylenimine, was formed. A cellulose base sheetwas softened with this composition. The sheet was dried and coated witha lacquer composition as described above, and the resultant coated sheetwas tested by immersion in water at 75 C. The coating did not disengagefrom the base sheet for a period of two-hours.

Example V A solution was formed by dissolving 4.3 grams of a 50% aqueoussolution of polyethylenimine in 17.2 grams of water at room temperature.To this solution 8.1 grams of 37% formaldehyde in water was added; andthe solution formed thereby was employed to form a softener bathcontaining less than 1.0% by weight of solids and 10% by weight ofglycerol. 'It was 'found that this solution could be used successfullyto soften a regenerated cellulose f base sheet so that whenanitrocellulose coating was subsequently formed thereon, as describedabove, it was not detached from the base sheet after immersion in waterfor a period of two hours.

From the foregoing, it is apparent that a versatile and effective methodof anchoring nitrocellulose coatings to cellulosebase sheets isdisclosed. It is further apparent that novel compositionsof matter arealso taught. It will be understood that alternative materials may beused in place of a number of the ingredients disclosed and that changesin the concentrations of the ingredients may be made without departingfrom the scope of the subject invention.

Anorganic resinous surfacecoating formed from an organic lacquer mayhave the following approximate composition:

Percent Nitrocellulose 40 to 60 Plasticizer 30 to 40 Blending resin 3 to15 Wax 1 to 5 Polymericplasticizer 5 to 15 With regard to the aldehydewhich is reacted with the ethylenimine polymer, it will beunderstoodthat although formaldehyde is preferred for this purpose, otheraldehydes such as polyaldehyde, acetaldehyde and the like may also beused or on mixtures of such other aldehydes may be employed withformaldehyde.

Where the reaction product of the aldehyde and ethylenimine polymer isto be employed in a solution, as for example in a softener bath, it ispreferred that it be formed in such manner as to limit the formation ofthe product in gel or dispersion form in the bath.

Where the reaction product is to be employed in a coating composition,such as in a lacquer coating composition or in an aqueous dispersioncoating composition, the formation of the gel-like product maybesatisfactory.

With regard tothe softener bath composition, the glycerol softener maybe replaced by other suitable polyhydric alcohol softeners, as the needsof the softener bath and content of the regenerated cellulose sheet maydictate. 7

With particular regard now to the base sheet impregnated with thiscomposition, a base sheet of regenerated cellulose is preferred althoughnumerous other base sheets, such as a sheet of cellulose nitrate orother hydrophilic base sheets, may be employed.

Whereas the foregoing examples are given with particular reference tothe coating of a base sheet with nitrocellulose, it will be understoodthat other coatingsrmay be employed. For example, a coating ofpolymerized vinylidene chloride, dispersed in an aqueous medium, may beemployed. Such coatings should preferably either contain an acid mediumsuch as acrylic or methacrylic acid or the like, or should have asufliciently low pH value so that the coating becomes strongly anchoredto the base sheet.

A number of lacquer. solvents such as cyclohexanone, tetrahydrofur-an,mixtures of ethyl acetate and toluene lower ketones and the like may beused. It is preferred to include an acid condensation catalyst in thelacquer as indicated above.

A polyethylenimine product found suitable for carrying out the subjectmethod is a polymer product available from Hansborg and Company of NewYork. Polyethylenimines available from other sources may also be used.

One particular advantage of the subject method is the ease ofincorporating the novel reaction product of formaldehyde andpolyethylenimine into the film. As noted above, it can with greatfacility be incorporated either into the base sheet or into the coatingcomposition.

This advantage is further illustrated by the fact that the reaction-canbe brought about at the time the base sheet and coating composition arebrought into contact and formed into a film. Thus, the formaldehydereactant may be incorporated in the base sheet and'the polyethy leniminemay be incorporated in the coating. The two are brought onto contact andthe reaction of these two reactants takes place. Heating of the film toevaporate thecarrying medium from the composition and to consolidate thecoating layer enhances the conditions for reaction. Excess formaldehydeis volatile and is removed during the drying treatment.

Conversely, the formaldehyde may be incorporated into the coatingcomposition, particularly a latex coating composition and thepolyethylenimine may be incorporated in the base sheet. Reaction ofthese two reactants occurs when the base sheet and composition arebrought together and treated to form them into a film.

These alternative procedures further illustrate the ease and facility ofimparting improved properties to films in accordance with the subjectmethod.

Since many examples of the foregoing procedures, compositions, andarticles may be carried out and made, and since many modifications canbe made in the procedures and articles described without departing fromthe scope of the subject invention, the foregoing is to be interpretedas illustrative only, and not as defining or limiting the scope of theinvention.

We claim:

1. The method of anchoring a nitrocellulose-base surface coating to acellulose base sheet which comprises impregnating said base sheet withthe reaction product of formaldehyde and polymerized ethylenimine,drying said sheet and applying to the surfaces thereof a lacquer coatinghaving an organic acid condensation catalyst and over 30 percent ofnitrocellulose.

2. The method of anchoring an organic resinous surface coating to aregenerated cellulose base sheet, which comprises incorporating thereaction product of an aldehyde and polymerized ethylenimine into saidbase sheet, forming an acidic organic lacquer composition containing anorganic acid condensation catalyst and said organic resinous substance,applying said lacquer to said sheet and heating to evaporate the solventfrom said lacquer and to bond the resultant surface coating to said basesheet.

3. The method of claim 2 wherein the aldehyde is formaldehyde.

4-. The method of claim 2 wherein the organic resinous substance has thefollowing approximate composition:

5. The method of anchoring an organic resinous surface coating to aregenerated cellulose base sheet which comprises forming a softener bathby adding about 0.2% by weight of polyethylenimine and about 0.3% byweight of formaldehyde to an aqueous solution of a po lyhydric alcoholsoftener to form the reaction product of polyethylenimine andformaldehyde in said bath, softening said sheet in said softener bath,coating said sheet with a lacquer composition containing an organic filmforming resin in an organic solvent and an organic acid condensationcatalyst, and heating said sheet to evaporate said solvent and to bondsaid resin to said sheet.

6. The method of claim 5 wherein the lacquer composition contains anorganic acid of low pH value.

7. The method of claim 5 wherein the acid lacquer composition containsan acid selected from the group consisting of itaconic, acorritic,citric, lactic, acrylic, and methacrylic and the resin isnitrocellulose.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD OF ANCHORING A NITROCELLULOS-BASE SURFACE COATING TO ACELLULOSE BASE SHEET WHICH COMPRISES IMPREGNATING SAID BASE SHEET WITHTHE REACTION PRODUCT OF FORMALDEHYDE AND POLYMERIZED ETHYLENIMINE,DRYING SAID SHEET AND APPLYING TO THE SURFACES THEREOF A LACQUER COATINGHAVING AN ORGANIC ACID CONDENSATION CATALYST AND OVER 30 PERCENT OFNITROCELLULOSE.